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Adding comments

Write the comment flow with addComment.

Linking comments to a post

To add comments to a blog post, we extend the Post class with a List<Comment> comments field. Each post then carries its own comments. The addComment(Comment c) method appends a new comment to the list.

This design creates a composition between Post and Comment: a Post is made of Comments. Comment stays an independent class; one Comment can be reused on several posts.

The addComment method

The addComment method works like addPost: it appends the comment with comments.add(c);. You call it with post.addComment(new Comment(...)).

Since each Post keeps its own comment list, comments added to different posts do not mix. Use post.comments.size() to learn how many comments a post has.

Practice Add a comments field and addComment to Post

Add a List<Comment> comments = new ArrayList<>(); field to the Post class and a void addComment(Comment c) method that calls comments.add(c);. In main, create a Post p = new Post("First", "Ayşe");, add a comment with p.addComment(new Comment("Mehmet", "Nice"));, and print the comment count with System.out.println(p.comments.size());.

Starter code

Main.java
import java.util.List;
import java.util.ArrayList;

class Post {
    String title;
    String author;
    Post(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

class Comment {
    String author;
    String text;
    Comment(String author, String text) {
        this.author = author;
        this.text = text;
    }
}

// Add comments field and addComment method to Post

// main: create a Post, add a comment and print the count

Sample solution

Main.java
class Post {
    String title;
    String author;
    List<Comment> comments = new ArrayList<>();
    Post(String title, String author) {
        this.title = title;
        this.author = author;
    }
    void addComment(Comment c) { comments.add(c); }
}

class Comment {
    String author;
    String text;
    Comment(String author, String text) {
        this.author = author;
        this.text = text;
    }
}

public class Main {
    public static void main(String[] args) {
        Post p = new Post("First", "Ayşe");
        p.addComment(new Comment("Mehmet", "Nice"));
        System.out.println(p.comments.size());
    }
}

Expected output

1

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What kind of relation is the Post-Comment link?

  • Composition Correct answer
  • Inheritance
  • Dependency
  • Abstraction

Explanation: A Post is made of Comments; this is composition.

2 Match the Post-Comment relation

Term Meaning
composition Post contains its Comments
List<Comment> Comment collection type
addComment Adds a comment
comments.size() Number of comments

3 Complete the add-comment call

Question 
p.add____(new Comment("Mehmet", "Nice"));
Answer 
p.addComment(new Comment("Mehmet", "Nice"));

Explanation: The addComment method appends a new comment to the list.

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. new
  5. toString
  6. Summary - Classes

2 Constructor

  1. Constructor basics
  2. Parameterized constructor
  3. Constructor overloading
  4. this()
  5. Default constructor
  6. Summary - Constructor

3 Inheritance

  1. extends
  2. super
  3. Method overriding
  4. Object class
  5. final class and method
  6. Summary - Inheritance

4 Project: Library

  1. Class design
  2. Book model
  3. Member management
  4. Borrow flow
  5. Putting it together

5 Encapsulation

  1. private fields
  2. Getter
  3. Setter
  4. Package-private
  5. Record
  6. Summary - Encapsulation

6 Static

  1. static fields
  2. static methods
  3. static block
  4. Class variables
  5. instanceof
  6. Summary - Static

7 Constants

  1. static final
  2. Uppercase convention
  3. Constants with enum
  4. Interface constants
  5. Math constants
  6. Summary - Constants

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Interface

  1. interface
  2. implements
  3. default methods
  4. static methods
  5. Multiple interfaces
  6. Summary - Interface

10 Abstract class

  1. abstract class
  2. abstract methods
  3. Abstract and concrete
  4. Abstract vs interface
  5. Sealed class
  6. Summary - Abstract class

11 Polymorphism

  1. Polymorphism basics
  2. Dynamic binding
  3. Override rules
  4. Abstract reference
  5. Pattern matching
  6. Summary - Polymorphism

12 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

13 File I/O

  1. File class
  2. FileReader and FileWriter
  3. BufferedReader
  4. PrintWriter
  5. try-with-resources
  6. Summary - File I/O

14 Serialization

  1. Jackson ObjectMapper
  2. readValue
  3. writeValueAsString
  4. JsonProperty
  5. List and Map serialization
  6. Summary - Serialization

15 Generics

  1. Generic class
  2. Generic method
  3. Bounded type
  4. Wildcard
  5. Type erasure
  6. Summary - Generics

16 Generic collections

  1. List<E>
  2. Map<K,V>
  3. Set<E>
  4. Queue and Deque
  5. Collections class
  6. Summary - Generic collections

17 Iterator

  1. Iterator
  2. remove
  3. ListIterator
  4. Iterable
  5. Spliterator
  6. Summary - Iterator

18 Lambda expressions

  1. Lambda basics
  2. Parameterized lambda
  3. Method reference
  4. forEach with lambda
  5. Comparator with lambda
  6. Summary - Lambda expressions

19 Stream API basics

  1. stream()
  2. filter
  3. map
  4. forEach and count
  5. collect toList
  6. Summary - Stream API basics

20 Intermediate final

  1. Project design
  2. OOP model
  3. Stream layer
  4. File I/O
  5. Putting it together